The present invention relates to fluidic valves of the type used to control flow of a fluid. More specifically, the present invention relates to pulse width modulated control of such fluid flow.
Fluidic valves have many applications and are generally used to control flow of a fluid between two locations. One simple valve configuration is a simple blocking element positioned in a pipe, or the like, which can be moved between at least two positions. In one position, fluid is allowed to flow through the pipe while in the other position, the blocking element seals or partially seals against the pipe and blocks or restricts flow of fluid. If multiple positions are available between the fully “on” position (with large opening) and the fully “off” position (completely closed), flow of fluid can be further controlled accordingly. Valves with adjustable partial openings are the most prevalent means of controlling the pressure or flow in a hydraulic circuit. However, flow through partially open valves induces pressure drops across the valve, and consequently throttling energy loss, given by the product of the pressure drop across the valve and the flow, is incurred. Thus, such throttling valves are inherently, inefficient.
On the other hand, valves with binary positions—fully on or fully off, are inherently more efficient, since pressure drop is small when it is fully open, and flow is cut off when it is fully close. Thus, throttling loss in either positions can be zero or very small. In order to allow such on/off valves to achieve variable flow, the valve can be pulsed on and off at different times during the operation of the system. One such mode of operation is via pulse width modulation (PWM). In a pulse width modulated valve, the valve is rapidly switched between the fully on position and the fully off position. By changing the relative duration that the valve is in either the fully on position or the fully off position to the total period of an on/off cycle, the average flow rate can be accurately controlled between a maximum flow rate and zero flow rate. Such pulse width modulated valves can be used in many applications, for example, in achieving variable displacement functions from fixed displacement pumps and motors.
One example pulse width modulated valve configuration uses an obstruction which is moved linearly in a flow conduit between a fully blocking or closed position and a fully open position. The linear driving element can be, for example, an electromagnetic solenoid, a piezoelectric actuator (such as a PZT) actuator or the like. A critical factor in the performance of a pulse width modulated or other binary on/off valve configurations is the time it takes to transition between the fully on state, and the fully off state. Since the valve is throttling the flow during transition, it induces inefficiency. In a PWM valve, the proportion of time the valve is in transition relative to the time when it is fully on or fully off should be small to be efficient. On the other hand, cycling time (which consists of the fully on, fully off, and transition times) should be small for responsiveness and for precision. Thus, a short transition time is required for both efficiency as well as responsiveness and precision.